Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. In recent years the drop size of ink jet printers has tended to become smaller and smaller, resulting in higher resolution and higher quality prints. The smaller drop size is accompanied by smaller nozzle openings in the inkjet printhead. These smaller nozzle openings are easier to plug and more sensitive to extraneous deposits which can affect both the size and placement accuracy of the ink jet drop. The composition of the ink formula is known to contribute to nozzle plugging, and for this, among other reasons, humectants, biocides and surfactants are usually added to ink jet inks.
It has been recognized that there is a need to maintain the ink ejecting nozzles of an ink jet printhead, for example, by periodically cleaning the orifices when the printhead is in use, and/or by capping the printhead when the printer is out of use or is idle for extended periods of time. The capping of the printhead is intended to prevent the ink in the printhead from drying out. There is also a need to prime a printhead before use, to insure that the printhead channels are completely filled with ink and contain no contaminants or air bubbles and also periodically to maintain proper functioning of the orifices. Maintenance and/or priming stations for the printheads of various types of ink jet printers are described in, for example, U.S. Pat. Nos. 4,855,764; 4,853,717; and 4,746,938. Removal of gas from the ink reservoir of a printhead during printing is described in U.S. Pat. No. 4,679,059. In U.S. Pat. No. 4,306,245 to Kasugayama et al., a liquid jet recording device provided with a cleaning protective means for cleaning and protecting an orifice is described. The cleaning protective means is provided at a reset position lying at one end of the scanning shaft of the device.
U.S. Pat. No. 5,250,962 to Fisher et al., describes a movable priming station for use with an ink jet printer having a printhead with a linear extended array of nozzles. The movable priming station includes a support capable of moving along the extended array of nozzles and a vacuum tube having a vacuum port adjacent to one end thereof. The support is controlled so that the vacuum port does not contact the nozzle containing surface of the printhead when the support is moved along the linear array of nozzles. U.K. Patent Application GB2203994 to Takahashi et al., describes an applicator for applying antiwetting compositions to the nozzle bearing face of a printhead of an ink drop printer. The printhead which reciprocates across the face of a platen is moved to one end of the platen where the applicator is placed. The applicator includes an extendable pad which wipes the face of the printhead.
Conventional continuous ink jet printing utilizes electrostatic charging "tunnels" that are placed close to the point where the ink drops are formed in a stream. In this manner, individual drops may be charged, and these drops may be deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes known as a "catcher") may be used to intercept the charged drops, while the uncharged drops are free to strike the recording medium. If there is no electric field present, or if the drop break off point is sufficiently far from the electric field (even if a portion of the stream before the drop break off point is in the presence of an electric field), then charging will not occur.
Inks for high-speed ink jet drop printers must have a number of special characteristics. Typically, water-based inks have been used because of their conductivity and viscosity range. Thus, for use in a jet drop printer the ink must be electrically conductive, having a resistivity below about 5000 ohm-cm and preferably below about 500 ohm-cm. For good fluidity through small orifices, the water-based inks generally have a viscosity in the range between 1 and 15 centiposes at 25.degree. C.
Beyond this, the inks must be stable over a long period of time, compatible with ink jet materials, free of microorganisms and functional after printing. Required functional characteristics include resistance to smearing after printing, fast drying on paper, and being waterproof when dried.
Problems to be solved with aqueous ink jet inks include the large energy needed for drying, cockling of large printed areas on paper surfaces, ink sensitivity to rubbing, the need for an anti-microbial agent and clogging of the ink jet printer orifices from dried ink an other adventitious contaminants.
The non-water component of ink jet inks generally serves as a humectant which has a boiling point higher than that of water (100.degree. C.). The ink liquid vehicle components, i.e., the water and the humectants, generally possess absorption characteristics on paper and evaporation properties allowing for the desired ink jet printing speed when the ink is to be used in an ink jet printing process.
Many ink jet ink formulation have been patented. U.S. Pat. No. 5,738,716 by Domenic Santilli, et al. issued Apr. 14, 1998 describes the preparation of ink jet inks by dispersing pigments in water.
U.S. Pat. No. 5,431,722 by Yoshiro Yamashita, et al. issued Jul. 11, 1995 discloses the use of a buffer to control the pH of ink jet ink.
U.S. Pat. No. 5,725,647 James G. Carlson, et al., issued Mar. 10, 1998 disclose pigmented inks with added humectants.
As the prior art shows, there are many components added to ink jet ink formulations in order to minimize the problems noted above.